Manufacture of nitric acid esters

ABSTRACT

540,050. Alkyl nitrates. STEVENS, A. H. (Sharples Solvents Corporation). March 29, 1940, No. 5722. Drawings to Specification. [Class 2 (iii)] Alkyl nitrates are produced by reacting in alcohol with nitric acid of 35-68 per cent. strength under a pressure of 20-650 mm., the nitrate being removed by azeotropicdistillation, water (or in the case of ethyl or methyl nitrate, the ester) being returned from the distillate or otherwise to the reaction vessel to prevent accumulation of ester or water. The invention is applicable to the esterification of dihydric alcohols. Urea is preferably added during the process, and the water to be returned may be added in the form of solvent or diluent of the nitric acid, urea or alcohol. Examples 1-11 describe the esterification of n-butyl, 2-chlorethyl, mixed amyl, secondary amyl, iso-amyl, n-amyl, n-hexyl, secondary hexyl, 2-ethoxyethyl, cyclohexyl, and octyl nitrates, using the reagents in the liquid phase. The alcohol may be supplied in the liquid or vapour phase. Another example describes the production of isopropyl nitrate, the alcohol being supplied in the form of vapour above the surface of the nitric acid. Specification 379,312 is referred to.

Sept. Il, 1942.

MANUFACTRE oF J. F. OLIN El AL NITRIC ACID ESTERS Filed Aug. 20, 1938 ToVncuur l FREDERICK PR1Tscu BY JOSEPH J. SCHQEFER.

iowa/afa. a. OWMTTORNEY Patented Sept. l, 1942 MANUFACTURE 0F NITRICACID ESTERS John F. Olin, Grosse Ile, and Frederick P. Fritsch,Wyandotte, Mich., and Joseph J. Schaefer, Philadelphia, Pa., assignorsto Sharples Chemicals Inc., a. corporation of Delaware ApplicationAugust 20, 1938, Serial No. 225,908

(Cl. 26o-467) 7 Claims.

This invention relates to a process for the preparation of nitrateesters from aliphatic alcohols. The nitration of aliphatic alcohols toobtain esters of nitric acid has long presented a problem ofconsiderable commercial importance. The potential importance of suchesters is very great since they are useful as intermediates in thepreparation of a wide variety of organic chemicals in addition topossessing properties that render them suitable for many purposes asultimate products. Among the elds of usefulness in which these compoundsshow greatI promise, that of ignition promotion in Diesel engines isbelieved to be highly important. For example, a relatively smallquantity of an alkyl nitrate improves the combustion characteristics ofDiesel fuel to a remarkable degree.

Heretofore, the development of uses for compounds of the alkyl nitratetype has been hampered by the lack of a process for producing suchmaterials on a commercial scale at a reasonable price. Although it iswell known that nitric acid reacts with the alcohols to form nitrates,no really satisfactory method for conducting the reaction has beenheretofore developed. Under present practice the yields of nitratedproducts, with very few exceptions, are loW and the operations aredangerous. l

Two procedures for the preparation of nitrates from methyl, ethyl,propyl, butyl and amyl alcohols have been described. One method, whichhas been used to prepare nitrates from monohydric alcohols containingless than three carbon atoms, consists essentially in the distillationof a mixture of nitric acid and alcohol in the presence of urea. Thenitrate formed is readily volatilized and may be distilled from thereaction zone to prevent accumulation of the ester in the nitratingvessel which might otherwise result in explosions. Since the presence ofeven small amounts of nitrous acid tends to initiate a vigorousauto-oxidation of explosive violence, urea is added to decompose nitrousacid as formed in accordance with the following equation:

By the above process the methyl and ethyl nitrates may be prepared withfairly good results, but the method is not suitable for the nitration ofalcohols containing three or more carbon atoms. In the case of thenitration of propyl or butyl alcohol or alcohols of higher carboncontent than butyl alcohol, higher temperatures are required to obtainthe esterifcation reaction than when the lower alcohols are nitrated.Higher reaction occurring incident to nitration of the alcohol andtherefore preventing the successful practice of the nitration process.In such cases the oxidation of the alcohol can be to some eX- tentprevented by the addition of urea to the reaction mixture, but even whenthis precaution is taken the yield of the desired nitrate is still verylow. In the course of the oxidation reaction occurring when attempts aremade to nitrate the higher alcohols, the nitric acid intended foresterication is reduced to nitrous acid. The oxidation of the alcohol istherefore a deleterious reaction both from the standpoint of producingundesired -by-products and from the standpoint of reducing theesterifying acid. Even with the lowest alcohols the yieldl is rarely inexcess of 75% ("Systematic Organic Chemistry, Cumming, Hopper andWheeler, page 254) A process of nitrating isoamyl alcohol Vwhich hasbeen successful from a laboratory viewpoint involves the addition ofisoamyl alcohol-or a mixture of the alcohol with concentrated sulfuricacid to an ice-cold bath of nitric and concentrated sulfuric acid in theVolume ratio of 1:3. This process has not achieved commercial successbecause of the diiliculties of recovering unused nitric acid and ofreconcentrating the sulfuric acid. A more serious defect of this methodlies in the frequent explosions occurring unless the alcohol is verypure.

Neither of the above processes gives satisfactory results in thenitration of monohydric alcohols other than primary alcohols and nosatisfactory method has been heretofore devised for the preparation ofsecondary alkyl nitrates. The yields obtained by treating a secondary ortertiary aliphatic alcohol by either of the methods described above arenegligible. Further, ithas been found that the rst of these methods isunsuitable for the treatment of alcohols higher than ethyl.

It has been found that the use of dilute nitric acid avoids theobjections to prior processes to some extent. Again, using amyl alcoholas an example, it was found that moderate conversion of alcohol tonitrate was obtained by treatment with 35% to 60% nitric acid.Concentrations below 30% gave very low-grade esters while the use ofacid having a nitric acid content in excess of 60% resulted insubstantial oxidation with consequent decrease in yield. It has now beenfound that the objectionable features of prior art processes may beavoided to a large extent by lowering the pressure under which thenitration is accomplished and keeping the nitric acid dilute, and ofsubstantially uniform concentration by the process hereinafterdescribed.

An object of this invention has been the provision of a process forreacting an aliphaticl Experimental determinations indicate that amylnitrate and water form a constant boiling mixture at 20 mm. absolutepressure containing 1.8'parts of water to 1 part of ester and that mixedamyl alcohols distilled with water at the same pressure give a constantboiling mixture in which Water and alcohol are present in the approximate ratio of 125:1. On this basis it was calculated that -a 40% nitricacid solution should give satisfactory results at 20 mm. of mercuryabsolute pressure. In actual practice under such conditions it was foundthat the ratio of water to ester in the distillate was unexpectedlyhigh.

The present invention resulted from the form;- ulation of a theory thatappears to explain the facts and has led to the solution of the problem,although it is to be understood that the invention is not limited bythese theoretical concepts, which have yet to be conclusively proven.

It may be logically inferred that the esterication of an aliphaticalcohol proceeds through the formation of an oxonium derivative of verylow vapor pressure. From that intermediate compound, the ester is formedby loss of water, but in the presence of concentrated nitric acid, theoxonium derivative is oxidized to form undesired by-products. Thesereactions may be represented as follows:

l. CsHiiOH+HONO2=C5H11OH.HONO2 amyl alcohol oxonium derivative 2.C5H11OH.HONO2=C5H11ONO2+H2O oxonium derivative amyl nitrate water or 3.C5H11OH.HONO2=C4H9CHO+H`NO2+H2O oxouium derivative valeraldenitrous hydeacid In the above equations the reactions represented by Equations 1 and2 are the desirable reactions, while the reaction represented byEquation 3 is the undesirable reaction, since it results in theformation of undesired valeraldehyde and reduction of the nitric acidwhich might otherwise be converted into amyl nitrate.

When concentrated nitric acid isv used in the practice of the process ofnitration as indicated by Equations 1 to 3 above, the reactions ofEquations 1 and 3- predominate, and the desired reaction of Equation 2is depressed. 1

When concentrated nitric acid is used, the concentrated nitric acid alsocauses auto-oxida- Cil tion of the alkyl nitrate formed in accordancewith Equation 2 above, with the result that the contents of the reactionvessel frequently explode.

On the other hand, when very dilute nitric acid is employed in thisreaction, the reaction is extremely slow and the yields of the desiredproducts are small.

The operator, desiring to produce amyl nitrate, for example, is on .thehorns of a dilemma when attempting to operate in acocrdance with theprior art procedure for producing methyl and ethyl nitrates. If heinitiates the reaction with the use of a concentrated nitric acid, theinitial reaction involves undesired production of valeraldehyde andreduction of the nitric acid as indicated in Equation 3. If, on theother hand, he initiates the reactionby the use of nitric acid of theconcentration found most suitable for the accomplishment of thereactions of Equations 1 and 2, water formed incident to theesterication reaction as indicated in Equation 2 rapidly dilutes thereaction mixture and reduces the nitric acid to a concentration at whichit will not efficiently perform its nitrating function.

Reasoning from the above premises, the applicants conceived the presentinvention and solved the difficulties of the prior art procedures bydeveloping nitration technique whereby the concentration of the nitricacid in the esterication vessel may be maintained substantiallyconstant, and may be maintained at such point as to effectivelyaccomplish the nitration function while minimizing the occurrence of theundesired reaction of Equation 3, and minimizing the occurrence ofauto-oxidation of the alkyl nitrate formed in accordance with Equation2.

In accordance with therpresent invention, autooxidation in theesterication vessel is prevented by removing the alkyl nitrate from theesterication vessel promptly after the formation thereof and water isremoved with the alkyl nitrate in the form of an azeotropic mixture. Theamount of water necessary to maintain the desired dilution of the nitricacid in the esterication vessel may be added to the esterication vessel,when necessary, in the form of a diluent for the nitric acid and/oralcohol added to the esterication vessel from vtime to time and a. partof the water required for such dilution may be returned to theesterication vessel from the azeo- Vtropic mixture removed from thevessel by condensing and decanting this azeotropic mixture and returningthe water so decanted to the esterification vessel.

The azeotropic distillation of the mixture of water, alkyl nitrate,alcohol and nitric acid from the esterication vessel is accomplished inaccordance with the preferred embodiment of the invention by maintaininga sub-atmospheric pressurein that vessel. The maintenance of suchsub-atmospheric pressure enables the operator to remove the azeotropicmixture from the vessel without the application of excessive heat, andthus to maintain'the reaction mixture at a lower temperature than couldbe maintained if distillation were attempted at atmospheric pressure. yAs the result of the maintenance of this lower temperature, the reactionof Equations 1 and '2 is favored, the reaction of Equation 3 issuppressed, and auto-oxidation of the alkyl nitrate is suppressed.

Since there is usually a tendency toward formation of nitrous acid inthe reaction mass which will initiate oxidation if permitted toaccumuiate, it is preferred that a small proportion of urea be added tothe vessel in which the present process is practiced. As a general rulethe proportion of urea added is less than that employed in the processof the prior art, but as a precaution against decreased yields, inferiorgrade ester, and explosion it is always well to follow the practice ofadding 'at least a small amount of urea. The fact that only smallamounts of nitrous acid are formed in the reaction here described whenpracticed in accordance with the present invention is attested by thepresence of urea nitrate crystals remaining in the reaction vesselfollowing the nitration of alcohol by the process of this invention whenthe quantities of urea are those prescribed by the prior art.

In its preferred embodiment the invention may be advantageouslypracticed in the apparatus shown diagrammatically in the accompanyingdrawing wherein I, 2 and 3 designate storage tanks for alcohol, 50% ureasolution and 70% nitric acid, respectively, each connected by a valvedpipe to reaction vessel 4. The reaction vessel is equipped with acondenser 6. A pipe to convey distillate connects condenser 6 to adecanter l, adapted to separate the distillate into two layers ofsubstantially immiscible liquids of different specific gravity. Thedraw-oir lines for the decanter are preferably adapted to be transposedin order that either the lighter or heavier liquid can be passed tostorage vessel 8, While the other layer is recycled, at least in part,to reaction vessel 4, through a line having an outlet controlled byvalve 9 to permit withdrawal of water from the system.

In operation of the described apparatus for continuous nitration, nitricacid of the desired concentration is supplied to reaction vessel 5,together with a. small quantity of vurea, the pressure reduced in thesystem to the desired point and heat applied to vessel until rapidboiling of the acid is begun. Molecular equivalents of aliphatic alcoholand nitric acid from tanks l and 3 are slowly run into vessel 0 at a xedrate and enough urea solution is added from tank 2 continuously orintermittently to substantially inhibit oxidation due to the presence ofnitrous acid. The vapors rising from the reaction mass pass through thecolumn 5 to condenser 6 from which a distillate containing alkylnitrate, alcohol, water and a little nitric acid is conducted todecanter 1. In the decanter, the distillate separates into two layers.The upper layer in most instances contains alkyl nitrate and alcoholwhich is passed continuously to storage vessel 8, from which it isWithdrawn for purification. The water layer may be used in part todilute the 70% nitric acid to the desired degree, but since Water isobtained as a result of the reaction, it is necessary to continuouslywithdraw a part thereof from the system through valve 9 in order tomaintain the quantity of water in the vessel i substantially constant.Ester is withdrawn from storage vessel 8 and purified.

While the ester-alcohol layer in the decanter is generally lighter thanthe water layer, it must be borne in mind that some esters such as 2-chloroethyl nitrate, are heavier than water, and the decanterconnections must be interchanged in such esterication operations.

Using the apparatus described in the manner set forth, a large number ofdifferent aliphatic nitrates have been prepared with uniformly goodresults.

EXAMPLE 1.-n-Butyl nitrate-Using the apparatus described above n-butylnitrate was prepared by the reaction of nitric acid on n-butanol. Thereactor was initially charged with 10 mols of a 50% aqueous solution ofnitric acid which was then vigorously boiled at a pressure of about 400mm. of mercury absolute, which pressure was maintained substantiallyconstant throughout the run. 20 mols of n-butyl alcohol and 22 mols ofnitric acid in the form of .a 70% solution were added during the courseof the experiment. The average temperature in the reaction vessel wasapproximately 97 C. Small quantities of 50% urea solution were addedfrom time to time in order to inhibit oxidation. The crude ester waswashed, neutralized, dried and fractionated at reduced pressure toobtain 17.3 mols of butyl nltrate and 1.23 mols of butyl alcohol.Approximately grams of material was lost in the process of purifying thecrude ester. The normal butyl nitrate was obtained in a state of highpurity as a clear water-white liquid of mild ethereal odor, having aspecific gravity of 1.032 at 20 C. and a boiling point of 133 C. at apressure of 29.62 inches of mercury. The conversion of butanol to butylnitrate was 86.5% of the original charge With a yield of 92.2% based onthe alcohol consumed during the process. It is significant that only 20grams (0.33 mol) of urea were needed to prevent oxidation throughout therun. This is in the ratio of 0.019 mol of urea consumed to each mol ofester formed. At the conclusion of the run 9 mols of nitric acidremained in the reaction vessel and 4.2 mols in the form of weak acidhad accumulated as withdrawals through valve 8 making a total of 13.2mols of acid recovered.

In the present example as in each of the following examples waterdecanted from decanter l was returned to the vessel d in suiiicientquantity to maintain the water content of the vessel t substantiallyconstant. Similarly, the alcohol and acid were continuously passed fromthe tanks l and 3 at a rate substantially equal to the rate at whichthese compounds were reacted together in the esterication vessel Il.Thus, the esteriflcation conditions were maintained substantiallyconstant during the entire course of the reaction by removal of anazeotropic mixture of ester and Water from the esterification vessel andfeed of water and acid and alcohol to the esterication vessel.

EXAMPLE 2 2-chloro ethyl nitrate.-In a similar manner, nitric acid wasreacted with a 38% aqueous solution of ethylene chlorohydrin. A pressureof 260 mm. of mercury was maintained. In this case the decanterconnections had to be reversed by reason of the fact that 2-chloro ethylnitrate is heavier than the weak acid condensed with' it. The latter,after purification, was obtained as a colorless liquid with a specicgravity of 1.388 at 20 C. and boiled at 46 C. under a pressure of 8 mm.of mercury. At atmospheric pressure the boiling point is approximately148 C. The product appeared to be contaminated by a trace of ethylenenitrate, since upon continuous distillation the small residue remainingin the still at 149 C. caused mild explosion.

EXAMPLE 3.-Mi:red amg/l nitrates-A mixture of isomeric primary andsecondary amyl' alcohols was nitrated. The original charge was 15.5 molsof nitric acid as a 50% solution under an absolute pressure of 300 mm.,48 mols of the mixed amyl alcohols were added to the boiling acid in thereactor together With 40 mols of nitric and 154.2o c.r

' isoamyl alcohol WBS nitrated at 350mm. of mer- C. with 50% vaporizingbelow 163 C.

gravity at 20'? C. was 0.976.

aiA

ity of 0.990 at C. and was a colorless liquid of fpleasant odor. .AnEngler distillation showed that 95% yof the mixture boiled between 145.8

l'lirxlimme 4.-*-Secondaru amy! ynitrates-4 mols of methyl propylcarbinol was reacted with nitric V acid 'in the manner described aboveat an absolute pressure of 300-mm.y of mercury and a temperature'of94.5' C. rThe reaction showed a rstrong f tendency toward oxidation,particularly in the j initial stages, which was readily controlled by fthe 'addition of urea.v Onthebasls of alcohol consumed, thefyield was87.2% rof water-white liquid having a mild ethereal odor with a.y specicy'gravityy of 0.992'fat 20 Gand boiling at 144 C. f

'Exmrrsr-Iso-amvl nitrate-A good cut of cury, employing the technique'ofthis invention with a yield of 92.6%. 0.043'mol' of yurea were used permol of ester produced. i f f f Exmrra Gif-Normal amylnitrat.e.---Peritanol-1y f was nitrated in ythe manner describedtogiv'er a 1.108 atj20 C. -Thepleasantodor is very muchY like that otbenzaldehyde. The product boiled l betweeny 68 and '12 C. at 18 mm. oimercury.

cinc gravity of 0.954 at 20 C., boilingv between 46 and 56 C. at 4 mm.of mercury. The reaction proceeded quite .smoothly butk a. little kmoref slowly than rin the preceding examples. 0.25

mol of 'urea per mol of alcohol were yused 'to ,i

inhibit oxidation.y y n y The examples given above are chosen as repfLresentativ'e of the operation of the process in its ypreferred form,but a large number of tests have f given results which indicate' thatthe inventionv e ymay be'g'enerally applied in the manufacture of alkylnitrates. v f

acid ysho d be ymaintained practically constant Forbes yresults theconcentration of the nitric ata value found to beA satisfactoryin'connection with the particular alcohol'being esteriiled. The

yield of 91.2% yunder ay pressure' of 250mm. ofy mercury. The lowproportion of urea'used (6.086l

other primary alcohols indicates :clearly that they f f primary alcoholsshow less tendency yto oxidation' during nitration than the secondaryyalcohols.y

The milicia was a colorless liquid boiling at 52 C. under 2 mm.'pressure and atfl'lfi C. under rpressure vwith 50% nitricr acid gave agood yield mol per-,mol of ester) wheny considered withl thev vsame datain 'connection' with 'the nitration of -oi the .present type. y acid asused in this disclosure and in the claims y -use of'concentrated acids,thatis. nitric acid conf taining substantiallyA more than y'10%' HNO:yis possible under very closely controlled conditions but should beavoided in most nltration reactions f The lterm dilute nitric appendedhereto isto be understood as referring vto an acid containing HNOinotsubstantially in excess of ,70%. f It has beeninoted that extremelynizraieQ-Nirrauon of ynormal hexyl nitrate yas a light colored liquid:40

oi' mild pleasant odor boiling at about 171 C. and having a. specificgravity' of 0.983 at 20 C.

Exnlrra 8.-Secondary hexyl nitrate- The nitration of methyl normal butylcarbinol emphasized again the greater tendency of secondary alcohols tooxidize. The control of that tendency required somewhat more urea thanwas used in connection with primary hexyl alcohol. The nnished ester wasobtained as a light yellow liquid, 90% ofwhlch boiled between 161 C. and165.8 Specific EXAMPLE 9.-2-Ethoxy ethyl nitrate.-Ethoxy ethyl alcohol(Cellosolve) was nitrated in accordance with the present invention withnitric acid and the pressure employed was 260 mm. The temperature of thereactor was 93 C. To prevent oxidation 0.07 mol of urea was used foreach mol of Cellosolve passed to the reactor. The finished product was acut obtained boiling between 59-65 C. at 15 mm. pressure, and was acolorless liquid. The Engler distillation showed 100% to dlstill between154.8160.8 C. As was the case with 2-chloro ethyl nitrate a mildexplosion occurred at the end of the distillation. Specinc gravity ofthe ester was 1.121 at 20 C.

Exner.: 10.-C'yclohexul nitrate.-Cyclohexyl alcohol was treated with 45%nitric acid under an average pressure of 300 nun. absolute. Because ofthe strong tendency of this secondary alcohol to oxidize, 0.42 mol ofurea. were used per mol of alcohol admitted to the reaction zone. Afterpurification the ester was observed to be a water-white liquid with aspecific gravity of dilute acids generally give yslow reactions `andpoor conversions and althoughsome of the ybenf f ents of this inventionmay be obtainedv with acid of ylow concentration itr is yusually foundthat better results are obtained when using rnitric f f acid yhaving anHNCh-'contentin excess 01,30%;

i. e., in'which; the total watercontent of the f esterlilca.tionr vesseldoes notbear ra. ratio to ther f total HNO: content of greaterv than70:30. While acids having a concentration between about 30% and 65% givehighly satisfactory results as compared with prior processes. thepreferred range is 45% to 55% of nitric acid since a reagent within thelatter limits has been found generally suitable to the nitration ofalcohols esteriiied in accordance with the present invention. A fairlygood general rule is that lower acid concentrations should be used inconnection with alcohols having a greater number of substituents on thecarbinol group; e. g., that lower acid concentrations should be used inesterifying secondary alcohols than primary alcohols.

In connection with most of the tests run, the acid in storage tank 3 hascontained 30% of water (specic gravity of 1.42). As discussed above,water, preferably that decanted from the raw product in order toconserve the acid content thereof, is added to the 70% acid in suchratio as to dilute the strong acid to the desired concentration. By thepreferred continuous process described above, a nitric acid solution isplaced in the reaction vessel and heated to vigcrous boiling underreduced pressure. Alcohol and nitric acid are then continuously added tothe heated mass in substantially molecular proportions. As the esterdistills over, a portion of the water in the distillate (containing somenitric acid) is decanted therefrom and used to dilute the acid suppliedto the reaction. As the reaction proceeds, water equivalent to thatformed by the nitration plus the amount used to dilute the acid as addedis Withdrawn from the system as a lay-product inthe form of dilutenitric acid.

'I'he contents of the reaction flask should be maintained in an activestate of ebullition and heat supplied at such a rate as to strip thealcohol and ester from the reaction mass. It is extremely important thatlarge quantities of these materials should not be -permitted toaccumulate in contact with the hot acid. If this precaution isignored,-excessive oxidation occurs, resulting in loss of yield, lowgrade product and explosions. i, The use of urea is recommended toinhibit oxidation. The process here described represents la strikingimprovement over the prior art by4 de- .pressing side reactions to.thevery minimuml and the amount of urea required is therefore very small.The low cost of this reagent in the amounts used in the preferredembodiment of this invention more than justies the additional safety andimproved yield obtained thereby. In connection with the nitration ofprimary alcohols, such as butanol-1, very little of the oxidationinhibitor is normally used, While more highly substituted :carbinolsgenerally require appreciable amounts of this reagent.

The absolute pressure permissible may be any subatmospheric pressurealthough it is highly desirable to operate at pressures above 20 mm. ofmercury. In general, it has been determined that a-bsolute pressureswithin the range of 50 mm. to 650 mm. of mercury give good resultsalthough much better results are obtained by maintaining the pressure at250-400 mm. abso` lute. As the pressure is reduced below 250 mm. theconversion of alcohol to nitrate is decreased. Pressures above thepreferred range result in low yields, due to the increased tendencytoward oxidation.

It is to be understood that the specific exa-mples given above are notexha-ustive, but only illustrative of the practice of the process.

As used in the description of the invention and in the following claims,the term aliphatic hydroxy compound is to be understood as`contemplating those compounds in which an hydroxy radical is attachedto a carbon atom of an aliphatic compound or group. Thus the inventionincludes nitration of alkyl, alicyclic and heterocyclic alcohols, exceptin the case of heterocyclic compounds, those in which an hydroxylradical is joined to an aryl grou-p.

In the above description of the invention, special emphasis has -beenplaced upon the esteriiication of alcohols containing between three andeight carbon atoms. While the invention has particular merit -whenapplied to the nitration of alcohols Within this range, it is alsoapplicable to alcohols containing more than eight carbon atoms and tomethyl and ethyl alcohols. In

cases in which the ester and water are removed :from the estericationvessel in the iorm of an azeotropic mixture containing less water thanthat formed during esterification, the invention -may be practiced bycontinuously or intermittently returning ester from the decanter to theesteriiication vessel instead of returning water to the esterni-cationvessel as described in the albove illustrative examples. Thus, in theesterication of methyl alcohol, it will be desirable to return methylnitrate to the esterification vessel in order that the ratio of methylnitrate to water in the esteriiication vessel may be suihcient to eiectremoval of the water fromthe esterication vessel in the form of anazeotropic mixture. Such return of ester to carry off the water asformed in the form of an azeotropic mixture enables the operator toremove the Water as formed,

thereby maintaining the acid in the esterifcation vessel at the desireddegree of dilution. Alternatively, in cases in which the azeotropicmixture of the ester with water normally contains less water tha-n thatformed incident to esterification, an entraimng liquid, such as benzene,can be intermittently or continuously added to the esterification vesselto increase the ratio of Water to ester in the distillate.

While the invention has been described in connection with operationsinvolving addition of the esterifying acid and alcohol in the liquidphase, the alcohol may, in an alternative embodiment of the invention,be reacted with the acid in the vapor phase. Thus, for example, alcoholfrom the tank I may be vaporized and passed into the space above the.boiling nitric acid in the reactor 4, instead of adding this alcohol inthe liquid phase. The following is an example of such an operation:

EXAMPLE l2.-Isopropyl nitrate.l0 mols of nitric acid in the form of a50% aqueous solution are brought to boiling temperature in the reactionvessel under an absolute pressure of 275 mm. of mercury. To the contentsof the reaction vessel nitric acid and iso-propyl alcohol are then addedin molecular equivalents, the isopropyl alcohol ibeing preheated and-passed in the vapor phase into the space above the yboiling nitric acidin the reactor 4. During the course oaf the reaction the liquid in theesteriication vessel 4 is at a temperature of about 92 C., while thetemperature at the top of the column 5 is slightly higher, indicatingthat reaction has occurred in the vapor phase. From time to time, ureasolution is added in quantities sucient to inhibit oxidation. Upon4washing with water and sodium carbonate solution, and then drying anddistilling, iso-propyl nitrate is obtained as a :colorless liquid with aspecic gravity of 1.043 at 20 C. boiling at 102 C.

While the invention has been described above in connection withoperations in which the ratio of water to HNOa in the estericationVessel is substantially constant, it will be evident that the advantagesof the invention may be at least partially realized in connection withoperations in which such ratio uctuates materially within the limitsabove indicated. Thus, it is not vital tothe practice of the inventionthat water be returned to the esterication vessel in the amountsnecessary to keep the ratio offwater to HNO; exactly constant.

Similarly, the, invention is not necessarily limited to the practice ofesterication by a continuous process. tion can be partially realized byreturning water or ester to the esterication vessel in such proportionsas to reduce the degree of concentration or dilution of the acid in theesterication vessel and thereby avoid the alternative defects of theprior art of over-concentration or overdilution of the acid.

The advantages of the invention may also be partially realized in theesterication of some alcohols without the use of vacuum in the practiceof the azeotropic distillation. Thus, the features of reducing thetemperature in the esteriiication vessel by the employment of azeotropicdistillation and avoiding or reducing the tendency of the nitric acid tobecome more dilute or more concentrated as the case may be, as theesterication reaction proceeds, have an advantage independent of theadvantage obtained by the use of sub-atmospheric pressures in the Theadvantages of the invenesterication vessel, and these features may beused per se to obtain such advantage.

In the above description, considerable emphasis has been placed uponreturn of water from the decanter 'l to the esterication vessel 4. Wewish it to be understood that such return is not vital to the practiceAofthe invention. There is a certain advantage in 4returning water fromthe decanter 1 because of the fact that this water contains unreactednitric acid which may be utilized upon return of such water. However,the amount of water necessary to maintain the contents of theesteriiication vessel 4 at the desired degree of dilution may be addedto the esterification vessel in the form of diluent for the nitric acidin the container 3, or such water may be separately added to theesterication vessel.V

Thus, in its broadest aspects, the invention includes the addition ofwater whether in the form of diluent for the nitric acid, alcohol orurea added during the course of the reaction, in the form of dilute aciddecanted from the decanter 1 or in the form of separately added water.

Still further modiiications will be obvious to those skilled in the artand we do not therefore wish to be limited except by the scope of thesub-joined claims.

We claim:

1. The process of preparing an alkyl nitrate containing between 3 and 8c rbon atoms in its alkyl radical comprising conta ting boiling dilutenitric acid with a monohydric aliphatic alcohol containing the alkylradical of the desired alkyl nitrate under a pressure between 20 and 650mm. of mercury, continuously removing from the esteriiication zone waterand the desired alkyl nitrate by azeotropic distillation during thecourse of the esterincation reaction. and adding water to theesterication zone during the course of the esteriiication reaction tomaintain the ratio of water to nitric acid in the esterincation zone ata concentration between and 68% during the course of said reaction.

2. The process of preparing an alkyl nitrate containing between 3 and 8carbon atoms in its alkyl radical comprising contacting boiling dilutenitric acid with a monohydric aliphatic alcohol containing the alkylradical of the desired alkyl nitrate under a pressure between 20 and 650mm. of mercury, continuously removing from the esterication zone waterand the desired alkyl nitrate by azeotropic distillation during thecourse of the esterication reaction, decanting the azeotropic mixtureremoved from the esterification zone to obtain a decanted fractioncontaining the water and a second decanted fraction containing thedesired alkyl nitrate, and returning said decanted water fraction to theesteriiication zone to maintain the ratio of water to nitric acid in theesteriilcation zone at a concentration between 30 and 68% during thecourse of said reaction.

3. 'Ihe process of preparing an aliphatic nitrate containing at leastthree carbon atoms in the molecule which comprises treating a monohydricaliphatic alcohol containing between 3 and 8 carbon atoms in themolecule with nitric acid having a concentration between 35% land 68%and removing the resulting aliphatic nitrate by azeotropic distillationunder a reduced pressure between 20 and 650 mn. of mercury, and addingto the reaction mixture water in sufilcient amount to prevent anaccumulation of ester in the reaction mixture during the course of theesteriflcation reaction and maintain the nitric acid in theesterification reaction at a concentration between 35% and 68%.

4. The process of preparing an alkyl nitrate containing between 3 and 8carbon atoms in its alkyl radical comprisingcontacting boiling dilutenitric acid with a monohydric aliphatic alcohol containing the alkylradical of the desired alkyl nitrate, and continuously removing from theesterication zone water and the desired alkyl nitrate by azeotropicdistillation during the course of the esteriiication reaction whilemaintaining the concentration of the nitric acid between 35 iand 60% byadding water to the reaction mixure.

5. The process of preparing an alkyl nitrate containing between 3 and 8carbon atoms in its alkyl radical comprising contacting boiling dilutenitric acid with a monohydric aliphatic alcohol containing the alkylradical of the desired alkyl nitrate. continuously removing from theesterilication zone water and the desired alkyl nitrate by azeotropicdistillation during the course of the esteriiication reaction, andreturning water of the esterification reaction to maintain the nitricacid in the esteriilcation zone at a degree of concentration between 35and 60%.

6. The process of preparing an aliphatic nitrate containing at leastthree carbon atoms in the molecule which comprises treating a monohydricaliphatic alcohol containing between 3 and 8 carbon atoms in themolecule with nitric acid having a concentration between 45% and 55% andremoving the resulting aliphatic nitrate by azeotropic distillationunder a reduced pressure between 20 and 650 mm. of mercury, and addingto the reaction mixture water in suilicient amount to prevent anaccumulation of ester in the reaction mixture during the course of theesteriiication reaction and maintain the nitric acid in theesterification reaction at a concentration between 45% and 55%.

- 7. The process of preparing an aliphatic nitrate containing at leastthree carbon atoms in the molecule which comprises treating a monohydricaliphatic alcohol containing between 3 and 8 carbon atoms in themolecule with nitric acid having a concentration between 45% and 55% andremoving the resulting aliphatic nitrate by azeotropic distillationunder a reduced pressure be- -tween 250 and 400 mm. of mercury, andadding to the reaction mixture water in sufficient amount to prevent anaccumulation of ester in the reaction mixture during the course of theesteriiication reaction and maintain the nitric acid in theesterification reaction at a concentration between 45% and 55%.

JOHN F. OLIN.

FREDERICK P. FRITSCH.

JOSEPH J. SCHAEFER.

